1. Field of the Invention
The present invention relates to techniques for producing fibers from a thermoplastics material having a high melting point, for example of the glass or basalt type. More particularly, the invention relates to a development in so-called internal centrifugation methods of drawing out fibers in which the material, in the molten state, is poured into a centrifuge rotating at high speed, the periphery of which is provided with a vast number of orifices through which the material escapes in the form of filaments which are then broken and possibly drawn out by concentric gaseous currents emitted parallel with the axis of rotation of the centrifuge. The invention likewise relates to an application of the method involving drawing out of fibers from relatively hard glasses for which the temperature corresponding to a viscosity appropriate to fiber drawing is close to their devitrification temperature.
2. Description of the Related Art
Over and above a few production units employing purely aerodynamic fiber drawing methods, most mineral wool production is carried out by centrifugation. The first techniques developed at the beginning of the century operated by molten material being poured onto an element rotating at high speed, the molten material becoming detached from the rotating element and being partially converted to fibers. These fiber drawing techniques, still referred to as external centrifugation methods, may be carried out with any type of material, particularly with materials having a high melting point such as basaltic glasses, because the rotating means can be cooled by an internal circulation of water or may be made of a refractory material having no pierced orifices. Furthermore, drawing is virtually instantaneous which makes it possible to use materials having a very high rate of devitrification at temperatures close to the fiber drawing temperature.
Conditioning of the molten glass need not be very elaborate. In other words this method allows melting without plaining, and quite possibly with a few pockets of non-molten material and with a composition which is not strictly constant over any period of time. However, this freedom is bought at the expense of a deterioration in the quality of the fibers and therefore the method cannot be used as widely as would be desired. Furthermore, the stream of molten material which falls onto the centrifuge upsets the drawing conditions so that the treatment undergone by two adjacent filaments may vary widely, which is of course reflected in the final appearance of the product which thus may have a wide diversity of fibers.
Another drawback is the fact that external centrifugation always results in a high level of non-fibered material, which reduces the heat and sound insulating properties of the products and for a given insulation situation results in denser products in that they comprise a considerable proportion of particles which do not contribute to the insulating capacity. In addition, these unfibered materials render the wool dusty and rough to the touch. The result is that external centrifugation techniques are no longer used nowadays except for glass compositions having a very high melting point, classified as hard glasses, for which the fiber drawing range is particularly narrow.
For the "finer" and softer glasses, the fiber drawing techniques systematically employed involve the internal centrifugation outlined hereinabove which advantageously results in a virtual absence of non-fibered material, longer fibers which impart increased resilience to the end product due to better interlocking of the fibers and greater accuracy with regard to the diameter of the fibers produced.
But in order to carry out such internal centrifugation, it is vital that the glass exhibit a satisfactorily rheological behavior. In the first place, the glass must be able to assume such a state that it can be drawn out, the diameter of the centrifuge orifices being of the order of a millimeter or, at the finest, a few tenths of a millimeter while that of the fibers produced must be of the order of a few microns. The filaments which escape from the centrifuge must therefore be thinned out by a minimum factor of one hundred. If the temperature of the glass is too high or in other words if the glass is too fluid, the fibers cannot be drawn out and in the end, due to the surface tension, droplets and not fibers will form (drop resolution temperature).
To this first limitation on the definition of the level of formation must be added the problem of devitrification, in that the glass must not be placed under conditions where it crystallizes at a sufficiently high rate, taking into account its dwell time in the centrifuge, a period of time with no common measure with the contact time between the glass and the rotating means in the case of fiber drawing by external centrifugation. The range of working temperatures is therefore likewise limited by the liquidus temperature (temperature corresponding to zero crystallization rate for a glass which is in thermal balance), or rather according to the usage in this art, by the higher devitrification temperature (the temperature corresponding to complete dissolution of the crystals in 30 minutes, measured on a previously devitrified glass). Subsequently, therefore, we will employ the term "working range" to define the range of temperatures at which fiber drawing is possible.
With the glasses currently used in internal centrifugation, the upper devitrification temperature is below the temperature corresponding to the highest acceptable viscosity for fiber drawing, and therefore the range of working temperatures is not, or is only very slightly, reduced by the devitrification problems.
On the other hand, the situation is entirely different with, for example, basaltic glasses or other glasses which have a particularly high melting point. For these, the devitrification temperature is far higher than the temperature corresponding to the highest viscosity so that the working range is limited by the devitrification and drop resolution temperatures. And furthermore, the gap between these two temperatures is often far less than a hundred degrees or so and may even be 50.degree. C., whereas a soft glass has a working range of more than 200-250.degree. C.
Since furthermore the glass is melted at a temperature which is higher than the working temperatures, with a gap which becomes increasingly great the harder the glass is, which would tend to constitute an even greater complication of the problem, it therefore has to cool down during the operations which convey it from the furnace to the peripheral wall of the centrifuge. Thus, it is virtually impossible to work with a glass very accurately at a given temperature throughout its entire dwell time in the centrifuge and hitherto it has not been possible to treat this type of glass with an internal centrifugation technique capable of replacing the external centrifugation technique, the aforesaid drawbacks of which have nevertheless been known for a long time.
And furthermore, to this basic problem of rheological behavior are added other problems, these of a technological nature. Indeed, these glasses are particularly corrosive and it is therefore necessary, for manufacturing the centrifuge, to find a material which is capable of resisting chemical attack which becomes increasingly more rapid when the temperature and rates of flow are particularly high. Furthermore, the size of the centrifuges, generally between 200 and 1000 mm in diameter, the vast number of outlet orifices and the shape which is designed with a view to withstanding intense mechanical stresses due to the rotation and plastic flow during long periods of use, do not make it possible to envisage constructing the centrifuge from refractory alloys, for example platinum. Various refractory steels are known in the literature pertaining to this art but all those which are suitable from the mechanical point of view have a maximum temperature of use (over a long period) of around 1000.degree. C. while a temperature of 1100-1200.degree. C. would be desirable.